There's no such thing as GMO buckwheat, and there's really very little (if any) difference between "ancient" buckwheat and conventional buckwheat. It's a very marginal "grain" and it's received very little attention from breeders. In other words, I suspect all buckwheat seed is basically the same.
Just generally speaking, there's no need to be concerned about unwittingly buying any kind of GMO seed. If you're shopping conventionally, you can't hardly avoid GMO feed or food (and the fact certainly won't be advertised), but seed is where the seed companies make their money. Certainly if it's something you could save seed from the seed company is going to make you sign an agreement, agreeing not to save seed and recognizing their "proprietary rights", before you can even buy it. And whatever GMO traits the seed companies have added to the seed will be well advertised, because those traits are precisely what they're selling to the farmers/growers.
If you're concerned about supporting GMOS's, where you probably spend the most money supporting GMO's is at the gas pump (with ethanol from GMO corn), secondly on animal feed (pretty much any non-organic dairy, grain-finished (regular) beef, non-organic eggs, poultry, or pork, including the "pastured" pork and eggs from "free-range" hens that you buy at the local farmers' market or from you neighbor with 10 hens, and even "farmed" fish), thirdly on processed food ("vegetable" or corn oil or cottonseed oil, anything with those oils or with corn syrup or with non-corn sugar often made from GMO sugar beets, very probably your pet food), fourthly on clothes (from GMO cotton), and only rarely on anything you'd buy unprocessed (like whole grains) or in the produce section (occasionally summer squash, likely sweet corn, a good bit of papaya, and not much, if anything, else in the produce department.) Of course, all this continues to change, so avoiding GMO's without buying USDA organic (which I don't see as a real solution -- for one thing USDA organic farms use and depend on things like GMO soybean meal for fertilizer, not to mention manure and slaughterhouse byproducts from GMO-fed animals) will soon be impossible unless you grow it yourself or pay a similarly motivated (and honest) neighbor the very high/fair price of growing it on a non-industrialized scale and dealing with you directly.